Ultrahigh-temperature laser melting replicates microstructures of natural pseudotachylytes

Superheated (temperature >2100 °C) melt was produced at room conditions by instantaneous laser heating of peraluminous garnet-sillimanite-rich gneiss. This gneiss hosts natural pseudotachylytes recording shallow-crustal seismic faulting. During melt quenching, mullite and spinel microlites crystallized extensively, together with sub-micrometric faceted rims of new garnet around garnet clasts. Experimental microlites are comparable to those of the natural pseudotachylyte and consist of high-melting-point phases crystallizing at undercooling conditions during quenching following the same sequence. This observation calls for caution when using the mineralogy of pseudotachylytes to infer depth of faulting. The static experiments also show that damage by purely thermal shock is relevant.